In recent years, a cover glass constituted by a chemically strengthened glass is used to increase protection and beauty of a display device of mobile devices such as a mobile phone, a smart phone, a personal digital assistant (PDA), and a tablet terminal.
A chemically strengthened glass has a tendency that strength increases as surface compressive stress (value) (CS) and a depth of a compressive stress layer (DOL) increase. On the other hand, to maintain a balance with surface compressive stress, internal tensile stress (CT) is generated inside a glass, and as a result, CT increases as CS and DOL are large. When a glass having large CT cracks, the cracking manner is vigorous with a lot of fragments, and as a result, the risk that the fragments scatter increases.
In view of the above, for example, Patent Document 1 discloses the formula (10) indicating acceptable limit of internal tensile stress of a chemically strengthened glass, and discloses that a chemically strengthened glass in which the number of fragments scattered is small is obtained by adjusting the following CT′ even though strength of the chemically strengthened glass is increased. Internal tensile stress CT′ described in Patent Document 1 is derived by the following formula (11) using measurement values of CS and DOL′.CT′≤−38.7×ln(t)+48.2  (10)CS×DOL′−(t−2×DOL′)×CT′  (11)
Here, DOL′ corresponds to a depth of an ion exchange layer.